Known are TFT comprising an amorphous silicon film. Such conventional TFT are used essentially for constructing active-matrix circuits to be in active-matrix type, liquid crystal display devices.
However, amorphous silicon TFT are problematic in that their acting speed is low and that they could not be used in practicable P-channel devices.
Because of such their problems, amorphous silicon TFT are used almost nowhere except for active-matrix circuits.
As one means capable of solving the problems, known is a structure comprising a crystalline silicon film.
To obtain such a crystalline silicon film, a heating method is known at present, in which, however, films of good quality could not be obtained.
The present applicant has already developed a technique of obtaining a crystalline silicon film under heat, in which the crystallization of silicon under heat is promoted by a predetermined metal element.
Concretely, the method developed by the applicant comprises introducing a metal element such as typically nickel into an amorphous silicon film followed by heating the film to thereby convert it into a crystalline silicon film.
The crystallinity of the crystalline silicon film as obtained in that method is higher than that of the others as obtained by heating only.
However, in the method, the nickel element used remains in the crystalline silicon film formed, thereby having some negative influences on the characteristics of TFT comprising the film.
Concretely, one problem with TFT comprising the film is that the characteristics of TFT vary with the lapse of time to lower the reliability thereof.
Another problem is that the method requires the nickel addition and therefore the steps constituting the method are complicated.